Reverse trick modes on progressive video using special groups of pictures

ABSTRACT

The invention concerns a method ( 200 ) and system ( 100 ) for encoding a video signal. The method includes the steps of receiving ( 212 ) a progressive video signal, and encoding ( 214 ) the progressive video signal into at least one group of pictures having at least one prediction source picture and at least one non-prediction source picture. All the non-prediction source pictures are predicted from the prediction source picture such that no non-prediction source picture is predicted from another non-prediction source picture. The method also includes the step of, in response to a reverse trick mode command, altering ( 218 ) the display order of the group of pictures to permit the group of pictures to be displayed in a reverse order. Additionally, the method can include the step of modifying ( 220 ) at least the number of non-prediction source pictures in the group of pictures in response to the reverse trick mode command.

BACKGROUND OF THE INVENTION

1. Technical Field

The inventive arrangements relate generally to video systems and moreparticularly to video systems that record or play back digitally encodedvideo sequences.

2. Description of Related Art

Devices that facilitate the playback of video are gaining popularity intoday's consumer electronics marketplace. For example, many consumershave purchased digital video disc (DVD) recorders or players forpurposes of viewing previously recorded programs or recording theirfavorite programs. A DVD recorder or player typically contains a MovingPictures Expert Group (MPEG) decoder to decode the digitally encodedmultimedia data that is stored on the discs that the recorder or playerplays. The MPEG video signal to be decoded is comprised of a pluralityof groups of pictures (GOP), each of which typically contain an intra(I) picture, a plurality of predictive (P) pictures and a plurality ofbidirectional predictive (B) pictures.

If the digital video recorder or player is connected to certaintelevisions, the digitally encoded signal will be decoded by the digitalvideo recorder or player's MPEG decoder before being displayed on thetelevision. Significantly, however, many digital televisions (DTV)contain their own MPEG decoders. As such, if a digital video recorder orplayer is connected to a DTV, the video signal read from the disc isremotely decoded by the DTV's decoder. This type of decoder isconsidered a passive decoder in that the microprocessor in the digitalvideo recorder or player has no control over the decoder. Thisconfiguration can be referred to as a remote decoder system.

During playback of a video signal, some viewers may wish to performcertain trick modes. A trick mode can be any playback of video in whichthe playback is not done at normal speed or in a forward direction. Asan example, a reverse trick mode can be initiated to allow the viewer tolocate portions of video that have already been played and that theviewer may wish to view again. The reverse trick mode can be at a normalspeed or pictures in a GOP may be skipped to produce a fast-reversetrick mode. In addition, duplicates of the pictures in a GOP may beinserted in the GOP to generate a slow-reverse trick mode. To effectuatea reverse trick mode on an MPEG video signal, the decoder of the DVD maydecode the pictures in a GOP in a forward direction. Once these picturesare decoded, the decoder is instructed to display the pictures inreverse order and if necessary, to add duplicate pictures to the GOP orto skip pictures in the GOP.

A remote decoder system, however, is not particularly suited to performreverse trick modes. The reason for this drawback is that themicroprocessor of the digital video recorder or player cannot instructthe decoder to display the pictures in reverse. As such, a reverse trickmode in such an arrangement is typically limited to merely sending tothe decoder in a reverse order the I pictures in all or some of the GOPsof the video signal.

SUMMARY OF THE INVENTION

The present invention concerns a method of encoding a digital videosignal. The method can include the steps of receiving a progressivevideo signal and encoding the progressive video signal into at least onegroup of pictures having at least one prediction source picture and atleast one non-prediction source picture. All the non-prediction sourcepictures are predicted from the prediction source picture such that nonon-prediction source picture is predicted from another non-predictionsource picture.

In addition, the method can include the steps of recording theprogressive video signal to a storage medium and playing back theprogressive video signal. In response to a reverse trick mode command,the method also includes the step of altering the display order of thegroup of pictures to permit the group of pictures to be displayed in areverse order. The method can also include the step of modifying atleast the number of non-prediction source pictures in the group ofpictures in response to the reverse trick mode command.

In one arrangement, the prediction source picture can be an intrapicture. Further, at least a portion of the non-prediction sourcepictures can be bidirectional predictive pictures or predictivepictures. As an example, each of the bidirectional predictive picturescan be one-directional bidirectional predictive pictures.

In one aspect of the invention, the modifying step can include the stepof skipping at least one non-prediction source picture in the group ofpictures. Alternatively, the modifying step can include the step ofinserting in the group of pictures a duplicate of at least onenon-prediction source picture.

In another aspect, the skipped non-prediction source picture can be apredictive picture being the last picture in display order in the groupof pictures. In addition, the method can include the step of convertingan immediate prior non-prediction source picture in display order in thegroup of pictures into a predictive picture unless the immediate priornon-prediction source picture is a predictive picture.

In another arrangement, each of the prediction source picture and thenon-prediction source pictures can contain a display indicator, and themethod can further include the step of modifying the display indicatorof at least a portion of the prediction source pictures andnon-prediction source pictures to reflect an intended display order. Asan example, the display indicator can be a temporal reference field.Such a modification step can occur following the altering the displayorder step or the step of modifying at least the number ofnon-prediction source pictures in the group of pictures.

In another arrangement, following the altering step, the method caninclude the step of converting the last non-prediction source picture inthe altered group of pictures to a predictive picture unless the lastnon-prediction source picture in the altered group of pictures is apredictive picture. Moreover, following the altering step, the methodcan include the step of selectively converting to bidirectionalpredictive pictures the non-prediction source pictures in front of, indisplay order, the prediction source picture. The method can alsoinclude the step of performing the receiving, encoding and alteringsteps in a remote decoder system.

The present invention also concerns a system for performing a reversetrick mode. The system includes a processor for encoding a progressivevideo signal into at least one group of pictures having at least oneprediction source picture and at least one non-prediction sourcepicture. All the non-prediction source pictures are predicted from theprediction source picture such that no non-prediction source picture ispredicted from another non-prediction source picture. The system alsoincludes a decoder for decoding the progressive video signal.Additionally, in response to a reverse trick mode command, the processoris programmed to alter the display order of the group of pictures topermit the group of pictures to be displayed in reverse order. Thesystem also includes suitable software and circuitry to implement themethods as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of a system that can encode a video signalinto special GOPs and perform a reverse trick mode in accordance withthe inventive arrangements.

FIG. 1B is a block diagram of an another system that can encode a videosignal into special GOPs and perform a reverse trick mode in accordancewith the inventive arrangements.

FIG. 2 is a flow chart that illustrates a method of encoding a videosignal into special GOPs and performing a reverse trick mode using inaccordance with the inventive arrangements.

FIG. 3 illustrates an example of a special GOP in accordance with theinventive arrangements.

FIG. 4A illustrates the GOP of FIG. 3 in reverse order in accordancewith the inventive arrangements.

FIG. 4B illustrates the GOP of FIG. 4A with modified display indicatorsin accordance with the inventive arrangements.

FIG. 4C illustrates the GOP of FIG. 4B with a converted picture inaccordance with the inventive arrangements.

FIG. 4D illustrates the GOP of FIG. 4C with another converted picture inaccordance with the inventive arrangements.

FIG. 5A illustrates one example of skipping pictures in the GOP of FIG.4D in accordance with the inventive arrangements.

FIG. 5B illustrates an example of inserting duplicate pictures in theGOP of FIG. 4D in accordance with the inventive arrangements.

FIG. 5C illustrates another example of skipping pictures in the GOP ofFIG. 4D in accordance with the inventive arrangements.

FIG. 5D illustrates yet another example of skipping pictures in the GOPof FIG. 4D and modifying display indicators of any remaining pictures inaccordance with the inventive arrangements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A system 100 for implementing the various advanced operating features inaccordance with the inventive arrangements is shown in block diagramform in FIG. 1A. The invention, however, is not limited to theparticular system illustrated in FIG. 1A, as the invention can bepracticed with any other system capable of receiving a video signal,processing the signal and outputting the signal to any suitablecomponent, such as a display device. In addition, the system 100 is notlimited to reading data from or writing data to any particular type ofstorage medium, as any storage medium capable of storing digitallyencoded data can be used with the system 100.

The system 100 can include an encoder 110 for encoding an incoming videosignal, and a microprocessor 112 for instructing the encoder 110 toencode the video signal in accordance with various techniques, some ofwhich will be explained later. All or portions of the encoder 110 andthe microprocessor 112 can be considered a processor 114 withincontemplation of the present invention. The encoder 110 can be locatedin the same apparatus as the microprocessor 112 or, alternatively, canbe positioned in a device that is remote from the apparatus housing themicroprocessor 112. If the encoder 110 is remotely located, the encoder110 is not necessarily under the control of the microprocessor 112.

The system 100 can also include a controller 116 for reading data fromand writing data to a storage medium 118. For example, the data can be adigitally encoded video signal. The system 100 can also have a decoder120 for decoding the encoded video signal when it is read from thestorage medium 118 and transferring the decoded video signal to asuitable component, such as a display device. The decoder 120 can bemounted in the same apparatus containing the encoder 110 (if the encoder110 is not remotely located), the microprocessor 112 and the controller116 or, as will be described below, may be mounted in a separate device.

Control and data interfaces can also be provided for permitting themicroprocessor 112 to control the operation of the encoder 110 (as notedabove), the controller 116 and the decoder 120. Suitable software orfirmware can be provided in memory for the conventional operationsperformed by the microprocessor 112. Further, program routines can beprovided for the microprocessor 112 in accordance with the inventivearrangements

In operation, the encoder 110 can receive and encode an incomingprogressive video signal. As is known in the art, this type of videosignal is comprised of pictures that have been progressively scanned. Inaccordance with the inventive arrangements, the microprocessor 112 caninstruct the encoder 110 to encode the incoming video signal into one ormore GOPs that are particularly useful for performing trick modes.Examples of such GOPs will be presented below. The encoder 110 can thentransfer the encoded video signal to the controller 116, which canrecord the signal onto the storage medium 118. In the case where theencoder 110 is remotely located, the encoder 110 can encode the incomingprogressive video signal, but the encoding instructions are notnecessarily received from the microprocessor 112.

If the microprocessor 112 receives a playback command, themicroprocessor 112 can instruct the controller 116 to read the encodedvideo signal from the storage medium 118. The controller 116 cantransfer the signal to the microprocessor 112, which can send the signalto the decoder 120. The decoder 120 can decode the video signal andoutput the signal for display on a suitable device. If themicroprocessor 112 receives a trick mode command, the microprocessor 112can skip pictures in the GOPs, insert duplicates of the pictures intothe GOPs or cause the display of any combination of the pictures in areverse order.

As alluded to earlier, there may be some instances in which the decoder120 that performs the decoding step is located in a device separate fromthe apparatus containing the microprocessor 112. An example of such anarrangement is illustrated in FIG. 1B in which the decoder 120 is in adisplay device 122, separate from a multimedia device 124 that can housethe microprocessor 112. In this case, the decoder 120 may not be underthe control of the microprocessor 112. Such a system can be referred toas a remote decoder system. Nonetheless, trick modes may still beperformed in this system 100 in which the microprocessor 112 may alterthe display order of the pictures in the GOP prior to decoding to permitthe pictures to be displayed in a reverse order. In addition, themicroprocessor 112 may delete pictures or insert duplicates of thepictures in the GOP prior to it being decoded by the decoder 120 in thedisplay device 122. It is understood that the encoder 110 in this typeof system may be remotely located as well.

In either of the arrangements discussed in relation to FIGS. 1A and 1B,the GOPs created during the encoding process will facilitate efficientimplementation of a reverse trick mode. The overall operation of theinvention will be discussed in detail below.

Reverse Trick Mode on Progressive Video using Special Groups of Pictures

Referring to FIG. 2, a method 200 that demonstrates one way to perform areverse trick mode on a progressive video signal using special GOPs isillustrated. The method 200 can be practiced in any suitable systemcapable of encoding and decoding a video signal. The method 200 canbegin, as shown at step 210. At step 212, a progressive video signal canbe received. As noted earlier, a progressive video signal containspictures that have been progressively scanned. As shown at step 214, theprogressive video signal can be encoded into at least one GOP having atleast one prediction source picture and at least one non-predictionsource picture. In one arrangement, all the non-prediction sourcepictures can be predicted from the prediction source picture such thatno non-prediction source picture is predicted from anothernon-prediction source picture.

Referring to FIG. 3, an example of such a process is shown. In thisparticular arrangement, the video signal can be encoded into one or moreGOPs 300. The GOPs 300 are shown in display order. Each of the GOPs 300can include at least one prediction source picture 310 and at least onenon-prediction source picture 312. A prediction source picture is apicture in a GOP that is not predicted from another picture yet can beused to predict other pictures in the GOP. In addition, a non-predictionsource picture can be any picture in a GOP that can be predicted from aprediction source picture in that GOP.

As an example, the prediction source picture 310 can be an I picture,and the non-prediction source pictures 312 can be B and/or P pictures.Each of the non-prediction source pictures 312 can be predicted from theprediction source picture 310, which in this example correlates to eachof the B and P pictures being predicted from the I picture. Because Ppictures can serve as non-prediction source pictures 312, it should beapparent that a non-prediction source picture 312 is not limited topictures from which no other pictures can ever be predicted, such as Bpictures.

In accordance with the inventive arrangements, however, each of thenon-prediction source pictures 312 can be predicted from the predictionsource picture 310 only. In one arrangement, the B pictures can beone-directional prediction pictures such that the B pictures prior to,or in front of, the I picture (in display order) can be backwardpredicted from the I picture, and the B pictures behind the I picture(in display order) can be forward predicted from the I picture. Thesubscript numbers incorporated into the prediction source pictures 310and the non-prediction source pictures 312 can indicate the order inwhich each of these pictures will be displayed—relative to the otherpictures in the GOP—at a normal (forward) playback speed.

As noted earlier, the GOP 300 is shown in display order. Thetransmission order is slightly different in that the prediction sourcepicture 310, in this example picture I₃, can be transmitted to a decoderfirst followed by the non-prediction source pictures 312 that will bepredicted from the prediction source picture 310.

It is important to note that the invention is in no way limited to theseparticular GOPs 300, as they represent merely one example of a GOPstructure in accordance with the inventive arrangements. In fact, anyGOP in which all the non-prediction source pictures in the GOP can bepredicted from a prediction source picture in that GOP is withincontemplation of the inventive arrangements. Moreover, although only twoGOPs 300 are shown in FIG. 3 in which each GOP 300 has one predictionsource picture 310 and six non-prediction source pictures 312, it isunderstood that the received video signal can be encoded into anysuitable number of GOPs 300 having any suitable number of predictionsource pictures 310 and non-prediction source pictures 312.

Also, if more than one prediction source picture 310 is in the GOP 300,any B pictures in the GOP 300 can be bidirectionally predicted. As anexample, more than one prediction source picture 310 can be positionedin the GOP 300 and some of the non-prediction source pictures 312 can bepredicted from these prediction source pictures 310. As such, theprediction source pictures 310 can be transmitted to a decoder beforethe non-prediction source pictures 312 that are dependent on theseprediction source pictures 310 for their prediction.

Referring back to method 200, at step 215, the progressive video signalcontaining the GOPs can be recorded onto a suitable storage medium. Oncerecorded, the progressive video signal containing the GOPs can be playedback, as shown at step 216. At step 217, a reverse trick mode commandcan be received. In response, the display order of the GOP can bealtered to cause the GOP to be displayed in a reverse order, as shown atstep 218. An example of such a step is illustrated in FIG. 4A.

Here, each of the GOPs 300, as first depicted in FIG. 3, is shown withthe prediction source pictures 310 and the non-prediction sourcepictures 312 in reverse order. Altering the display order of thepictures in the GOP 300 may be helpful in performing a reverse trickmode, especially in a remote decoder system. The reason such a processis particularly useful in this type of system is because the decoder ina remote decoder system cannot receive instructions that direct it todisplay pictures in a reverse order. It is understood, however, that themethod 200 is in no way limited to application in a remote decodersystem.

The prediction source pictures 310 and the non-prediction sourcepictures 312 shown in FIG. 3 can contain display indicators. In onearrangement, the display indicator can be a temporal reference field. Atemporal reference field is typically a ten bit field located in thepicture header of digitally encoded pictures. Some decoders rely on thetemporal reference field to determine when a particular picture in avideo signal will be displayed relative to other pictures in the videosignal. This field normally has an integer value.

Referring back to FIG. 3, as an example, each GOP 300 contains sevenpictures. The subscript numbers for the pictures in each GOP 300 cancorrespond to the integer values for each respective picture's temporalreference field. For instance, the temporal reference field of the firstnon-prediction source picture 312, or picture B₀, can have an integervalue of zero, which indicates that this particular picture will be thefirst one in each GOP 300 to be displayed. The temporal reference fieldof picture B₁, the next picture to be displayed, can have an integervalue of one. Thus, the integer value of the temporal reference fieldfor each subsequent picture to be displayed can be higher by one, allthe way to picture P₆, whose temporal reference field can have aninteger value of 6. For convenience, the phrase “integer value of thetemporal reference field” can also be referred to as “integer value.”

When the display order of the pictures in the GOP 300 is altered topermit the GOP to be displayed in a reverse order, as shown in FIG. 4A,the original display indicators or integer values are no longer valid.As such, referring back to method 200 of FIG. 2, the display indicatorsof the prediction source pictures and the non-prediction source picturescan be modified to reflect an intended display order, as shown at step220.

An example of the result of this step is illustrated in FIG. 4B. Here,the new integer values that reflect the new display order are shown. Theoriginal integer values are shown in parentheses. Although the integervalue for the prediction source pictures 310 in this example did notchange, it must be noted that the invention is not limited in thisregard; it may be necessary, based on the GOP structure, to modify theinteger value of the prediction source picture 310 as well.

It is understood that the invention is not limited to this particularexample, as other ways to modify the integer values of the relevanttemporal reference fields to reflect an intended display order can beperformed in any other suitable fashion. Moreover, it should be notedthat the invention is not limited to the use of a temporal referencefield, as any other suitable display indicator can be modified toreflect an intended display order in either of the embodiments discussedabove.

Referring once again to method 200 of FIG. 2, at decision block 222, itcan be determined whether the last non-prediction source picture in thealtered GOP is a P picture. For purposes of the invention, the term“altered GOP” refers to a GOP in which the display order of the picturesin the GOP has been altered to permit the GOP to be displayed in reverseorder. If yes, the method 200 can resume at decision block 226 throughjump circle A. If it is not, the last non-prediction source picture inthe altered GOP can be converted to a P picture, as shown in step 224.An example of this process is shown in FIG. 4C.

The last non-prediction source picture 312 in the GOP 300, which wasoriginally picture B₆, has been converted to a P picture, or picture P₆.The reason for this conversion is that specifications for MPEG videorequire that the last picture in a GOP be a P picture or an I picture.As an example, a B picture can be converted to a P picture by setting toP picture values the following parameters located in the picture headerof the B picture: picture_coding_type; full_pel_backward_vector; andbackward_f_code. Additionally, the following variable length codes formacroblock_type can be set to P picture values: macroblock_quant;macroblock_motion_forward; macroblock_motion_backward;macroblock_pattern; macroblock_intra; spatial_temporal_weight_code_flag;and permitted spatial_temporal_weight_classes.

This step can instruct a decoder to decode the picture as a P picture.As such, in accordance with the inventive arrangements, the displayorder of a GOP can be altered to permit the GOP to be displayed in areverse order without violating the MPEG requirement that the lastpicture in a GOP be a P picture.

Referring back to method 200 of FIG. 2, at decision block 226 (from jumpcircle A), it can be determined whether all the non-prediction sourcepictures in the altered GOP that are in front of (in display order) theprediction source picture are B pictures. If they are, the method 200can continue at decision block 230. If they are not, then suchnon-prediction source pictures can be converted to B pictures, forexample, backward predicted B pictures, as shown at step 228.

For example, referring to FIG. 4D, following the alteration of thedisplay order, the first non-prediction source picture 312 was a Ppicture, or picture P₀, which is shown in parentheses. In accordancewith step 228, picture P₀ can be converted to picture B₀. In onearrangement, a P picture can be converted to a backward predicted Bpicture by setting by setting to B picture values the followingparameters located in the picture header of the P picture:picture_coding_type; full_pel_backward_vector; and backward_f_code.Additionally, the following variable length codes for macroblock_typecan be set to B picture values: macroblock_quant;macroblock_motion_forward; macroblock_motion_backward;macroblock_pattern; macroblock_intra; spatial_temporal weight_code_flag;and permitted spatial_temporal_weight_classes.

Because non-prediction source pictures 312 before (in display order) theprediction source picture 310 will be backward predicted pictures,converting such P pictures into B pictures improves the predictionscheme of the GOP 300, as P pictures cannot be backward predicted; theycan only be forward predicted. The prediction scheme to be used with thealtered GOP is shown in FIG. 4D.

Thus far, the GOP 300 has been described in relation to a reverse trickmode in which the pictures in the GOP 300 are displayed in a reverseorder at a normal playback speed (normal playback speed is 1×). Thereare, however, certain instances in which viewers may wish to view videoin reverse at speeds different from 1×, such as a fast-reverse orslow-reverse trick mode. Typically, the speed of video may be changed byeither adding pictures to or skipping pictures in the video.

Referring back to FIG. 2, it can be determined whether the number ofnon-prediction source pictures in the altered GOP are to be modified, asshown at decision block 230. If it is not, the method 200 can end atstep 242. If the number of pictures in the altered GOP is to bemodified, such a process can be performed at step 232. Several examplesare shown in FIGS. 5A through 5D.

Referring to FIG. 5A, each of the altered GOPs 300 (as illustrated inFIG. 4D) is shown with several non-prediction source pictures 312removed or skipped. Specifically, pictures B₀, B₂, B₄ and P₆ in the GOP300 on the left can be skipped, while pictures B₁, B₄ and P₆ in the GOP300 on the right can be skipped. Skipping such non-prediction sourcepictures 312 can cause the reverse playback speed to increase. Here, thenumber of non-prediction source pictures 312 skipped, one-half of allthe pictures in the two GOPs 300, correlates to a playback speed that istwice the speed of normal playback, or 2×.

In accordance with the inventive arrangements, any one of thenon-prediction source pictures 312 in the GOPs 300 can be skipped toincrease the reverse playback speed of the video signal withoutaffecting the prediction of any remaining non-prediction source pictures312 in the GOPs 300. This feature is made possible by the encodingprocess described above. A step for placing the GOPs 300 in accordancewith the MPEG standard, for example, will be discussed later.

Of course, it is understood that the invention is not limited to theexample described in relation to FIG. 5A, as the ability to skip allnon-prediction source pictures 312 in any order during a fast-reversetrick mode applies to any other GOP in which the non-prediction sourcepictures 312 are predicted from a prediction source picture 310. Also,the entire GOP 300 may be skipped to produce a faster playback.

Referring back to FIG. 2, the modifying step 232 can also include thestep of inserting in the altered GOP a duplicate of at least oneprediction source picture or non-prediction source picture to produce aslow-reverse trick mode. An example of such an operation is shown inFIG. 5B. Here, a duplicate of each prediction source picture 310 andnon-prediction source picture 312 can be inserted into the altered GOP300 (for convenience, only one GOP 300 is shown). This particularexample can produce a playback speed of ½×. The subscript letter “d”represents the picture to which it is associated as a duplicate of theimmediate preceding picture.

Similar to the original non-prediction source pictures 312, theduplicates of such pictures may be predicted from a prediction sourcepicture 310 (in accordance with the MPEG standard, the last picture inthe GOP 300, duplicate picture P_(6d), can be predicted from theimmediate prior P picture, which in this case is picture P₆). Inaddition, the original non-prediction pictures 312 and their duplicatesmay be predicted from the duplicate of a prediction source picture 310.

The example presented in FIG. 5B is explained as follows: all thenon-prediction source pictures 312 and their duplicates in front (indisplay order) of the original prediction source picture 310, or pictureI₃, may be predicted from picture I₃. Additionally, the originalnon-prediction source pictures 312 and their duplicates behind (indisplay order) the duplicate of the original prediction source picture310, or picture I_(3d), may be predicted from duplicate picture I_(3d)(with the exception of duplicate picture P_(6d)). It is understood,however, that this particular arrangement is merely an example, as thenon-prediction source pictures 312 and their duplicates can be predictedfrom any other suitable prediction source picture 310, including anyduplicate of a prediction source picture 310.

In another arrangement, one or more of the duplicate pictures insertedin the altered GOP 300 can be dummy B or dummy P pictures. A dummy B ora dummy P picture is a B or P picture, respectively, in which the dummypicture's motion vectors are set to zero and its residual signal is setto zero or not encoded. For example, the duplicate of the predictionsource picture 310 (picture I₃) in the altered GOP 300 can be a dummy Ppicture instead of another I picture, such as picture I_(3d). Similarly,the duplicate for the last non-prediction picture 312 (picture P₆) canbe a dummy P picture rather than a conventional P picture, such aspicture P_(6d). Using dummy B or P pictures during a trick mode canlower the bit rate of the video signal, which may be necessary incertain circumstances, particularly when the method 200 is beingperformed in a remote decoder system.

Referring back to FIG. 2, at decision block 234, it can be determinedwhether the last non-prediction source picture in the altered GOP hasbeen skipped. If no, the method 200 can resume at step 240. If yes, itcan be determined at decision block 236 whether the immediate priornon-prediction source picture in display order in the altered GOP is a Ppicture. If it is, the method 200 can continue at step 240. If it isnot, then the immediate prior non-prediction source picture in thealtered GOP can be converted into a P picture, as shown at step 238.

An example of this operation is illustrated in FIG. 5C. As notedearlier, the specifications for MPEG video require that the last picturein a GOP be a P picture. Thus, if picture P₆ in the altered GOP 300, anon-prediction source picture 312, were skipped during a fast-reversetrick mode, the last picture in the GOP 300 (if it is not skipped) wouldbe picture B₅, a violation of the MPEG standard. To satisfy the MPEGrequirement, the immediate prior non-prediction source picture 312, inthis case, picture B₅, can be converted into a P picture, or picture P₅.This conversion has been previously discussed, and it is unnecessary topresent it here. As such, the last picture in an altered GOP 300 can beskipped without violating the MPEG requirement that the last picture ina GOP be a P picture.

Referring back to the method 200 of FIG. 2, at step 240 (similar to step220) the display indicators of the prediction source pictures and thenon-prediction source pictures can be modified. Modifying the displayindicators of these pictures can reflect an intended display order ofthe altered GOP when any one of the prediction source or non-predictionsource pictures is skipped or duplicated.

When, for example, a non-prediction source picture is skipped, theprevious display order is no longer valid. Accordingly, the displayindicators of the prediction source pictures and the non-predictionsource pictures that follow the skipped picture can be modified toindicate a proper display order. This feature is also applicable ifduplicates of the prediction source pictures or the non-predictionsource pictures are inserted in the altered GOP.

As an example, referring to FIG. 5D, if picture B₁ in the altered GOP300 is skipped, then the integer values of the prediction sourcepictures 310 and the non-prediction source pictures 312 that follow thispicture can be decreased by a value of one. So, the integer value of thetemporal reference field of picture B₂ can be modified from two to one,the integer value of the temporal reference field of picture I₃ can bemodified from three to two and so on.

In this particular example, the new integer values are shown, theskipped picture B₁ is represented by a dashed outline and the oldinteger values are in parentheses. This modification process cancontinue until the end of the altered GOP 300 is reached and can ensurethat the remaining pictures in the altered GOP 300 will be displayed ina proper order. Each time a prediction source picture 310 or anon-prediction source picture 312 in an altered GOP is skipped, theinteger values of the temporal reference fields of the remainingpictures in that GOP that follow the skipped picture can be decreased bya value of one. Also, if pictures in the altered GOP are duplicated, theinteger values of the pictures that follow the added duplicates can beincreased by a value of one each time a duplicate is added. Referringback to FIG. 2, the method 200 can stop at step 242.

Although the present invention has been described in conjunction withthe embodiments disclosed herein, it should be understood that theforegoing description is intended to illustrate and not limit the scopeof the invention as defined by the claims.

What is claimed is:
 1. A method of performing a reverse trick mode,comprising the steps of: receiving a progressive video signal; encodingthe progressive video signal into at least one group of pictures havingat least one prediction source picture and at least one non-predictionsource picture, wherein all the non-prediction source pictures arepredicted from the prediction source picture such that no non-predictionsource picture is predicted from another non-prediction source picture;and in response to a reverse trick mode command, altering the displayorder of the group of pictures to permit the group of pictures to bedisplayed in a reverse order.
 2. The method according to claim 1,further comprising the steps of: recording the progressive video signalto a storage medium; and playing back the progressive video signal. 3.The method according to claim 1, further comprising the step ofmodifying at least the number of non-prediction source pictures in thegroup of pictures in response to the reverse trick mode command.
 4. Themethod according to claim 1, wherein the prediction source picture is anintra picture.
 5. The method according to claim 1, wherein at least aportion of the non-prediction source pictures are bidirectionalpredictive pictures.
 6. The method according to claim 1, wherein atleast a portion of the non-prediction source pictures are predictivepictures.
 7. The method according to claim 5, wherein each of thebidirectional predictive pictures is a one-directional bidirectionalpredictive picture.
 8. The method according to claim 3, wherein saidmodifying step comprises the step of skipping at least onenon-prediction source picture in the group of pictures.
 9. The methodaccording to claim 3, wherein said modifying step comprises the step ofinserting in the group of pictures a duplicate of at least onenon-prediction source picture.
 10. The method according to claim 8,wherein the skipped non-prediction source picture is a predictivepicture being the last picture in display order in the group of picturesand wherein said method further comprises the step of converting animmediate prior non-prediction source picture in display order in thegroup of pictures into a predictive picture unless the immediate priornon-prediction source picture is a predictive picture.
 11. The methodaccording to claim 1, wherein each of the prediction source picture andthe non-prediction source pictures contains a display indicator and themethod further comprises the step of modifying the display indicator ofat least a portion of the prediction source pictures and non-predictionsource pictures to reflect an intended display order.
 12. The methodaccording to claim 11, wherein the display indicator is a temporalreference field.
 13. The method according to claim 3, wherein each ofthe prediction source picture and the non-prediction source picturescontains a display indicator and the method further comprises the stepof modifying the display indicator of at least a portion of theprediction source pictures and non-prediction source pictures to reflectan intended display order.
 14. The method according to claim 13, whereinthe display indicator is a temporal reference field.
 15. The methodaccording to claim 1, following said altering step, further comprisingthe step of converting the last non-prediction source picture in thealtered group of pictures to a predictive picture unless the lastnon-prediction source picture in the altered group of pictures is apredictive picture.
 16. The method according to claim 1, following saidaltering step, further comprising the step of selectively converting tobidirectional predictive pictures the non-prediction source pictures infront of, in display order, the prediction source picture.
 17. Themethod according to claim 1, further comprising the step of performingsaid receiving, said encoding and said altering steps in a remotedecoder system.
 18. A system for performing a reverse trick mode,comprising: a processor for encoding a progressive video signal into atleast one group of pictures having at least one prediction sourcepicture and at least one non-prediction source picture, wherein all thenon-prediction source pictures are predicted from the prediction sourcepicture such that no non-prediction source picture is predicted fromanother non-prediction source picture; and a decoder for decoding thegroup of pictures; wherein the processor is further programmed to, inresponse to a reverse trick mode command, alter the display order of thegroup of pictures to permit the group of pictures to be displayed in areverse order.
 19. The system according to claim 18, further comprisinga controller for recording the progressive video signal to a storagemedium and playing back the progressive video signal.
 20. The systemaccording to claim 18, wherein the processor is further programmed tomodify at least the number of non-prediction source pictures in thegroup of pictures in response to the reverse trick mode command.
 21. Thesystem according to claim 18, wherein the prediction source picture isan intra picture.
 22. The system according to claim 18, wherein at leasta portion of the non-prediction source pictures are bidirectionalpredictive pictures.
 23. The system according to claim 18, wherein atleast a portion of the non-prediction source pictures are predictivepictures.
 24. The system according to claim 22, wherein each of thebidirectional predictive picture is a one-directional bidirectionalpredictive picture.
 25. The system according to claim 20, wherein theprocessor is further programmed to skip at least one non-predictionsource picture in the group of pictures.
 26. The system according toclaim 20, wherein the processor is further programmed to insert in thegroup of pictures a duplicate of at least one non-prediction sourcepicture.
 27. The system according to claim 25, wherein the skippednon-prediction source picture is a predictive picture being the lastpicture in display order in the group of pictures and wherein theprocessor is further programmed to convert an immediate priornon-prediction source picture in display order in the group of picturesinto a predictive picture unless the immediate prior non-predictionsource picture is a predictive picture.
 28. The system according toclaim 18, wherein each of the prediction source picture and thenon-prediction source pictures contains a display indicator and theprocessor is further programmed to modify the display indicator of atleast a portion of the prediction source pictures and non-predictionsource pictures to reflect an intended display order.
 29. The systemaccording to claim 28, wherein the display indicator is a temporalreference field.
 30. The system according to claim 20, wherein each ofthe prediction source picture and the non-prediction source picturescontains a display indicator and the processor is further programmed tomodify the display indicator of at least a portion of the predictionsource pictures and non-prediction source pictures to reflect anintended display order.
 31. The system according to claim 30, whereinthe display indicator is a temporal reference field.
 32. The systemaccording to claim 18, wherein the processor is further programmed to,following the alteration of the display order, convert the lastnon-prediction source picture in the altered group of pictures to apredictive picture unless the last non-prediction source picture in thealtered group of pictures is a predictive picture.
 33. The systemaccording to claim 18, wherein the processor is further programmed to,following the alteration of the display order, selectively convert tobidirectional predictive pictures the non-prediction source pictures infront of, in display order, the prediction source picture.
 34. Thesystem according to claim 18, wherein the processor and the decoder arepart of a remote decoder system.